Magnetic memory device

ABSTRACT

A magnetic memory device includes a magnetoresistive element and a first wiring layer. The magnetoresistive element includes a fixed layer, a recording layer, and a non-magnetic layer interposed therebetween. The first wiring layer extends in a first direction and generates a magnetic field for recording data in the magnetoresistive element. The recording layer includes a base portion extending in a second direction rotated from the first direction by an angle falling within a range of more than 0° to not more than 20°, and first and second projections projecting from the first and second sides of the base portion in a third direction perpendicular to the second direction. The third and fourth sides of the base portion are inclined with respect to the third direction in the same rotational direction as a rotational direction in which the second direction is rotated.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2005-207500, filed Jul. 15, 2005,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetic memory device, and moreparticularly to a magnetic memory device using a magnetoresistive effectelement as a memory cell.

2. Description of the Related Art

Various types of magnetic memories have been proposed so far. Further, amagnetic random access memory (MRAM) using magnetoresistive elementsthat exhibit a giant magnetoresistive effect (GMR) has recently beenproposed. In particular, attention is now being paid to an MRAM using aferromagnetic tunnel junction.

The ferromagnetic tunnel junction is formed of three layers, e.g., afirst ferromagnetic layer, an insulation layer and a secondferromagnetic layer. In this structure, a current flows through theinsulation layer by tunneling. The resistance of the junction varies inaccordance with the cosine concerning the angle between the direction ofmagnetization of the first ferromagnetic layer and that of the secondferromagnetic layer. Accordingly, the junction resistance assumes theminimum value when the direction of magnetization of the firstferromagnetic layer is parallel with that of the second ferromagneticlayer, and assumes the maximum value when the former is anti-parallelwith the latter. This is called a tunneling magnetoresistive (TMR)effect, and there is a case where the rate of change in the junctionresistance due to the TMR exceeds 70% at room temperature.

In a memory cell including a ferromagnetic tunnel junction, at least oneferromagnetic layer is regarded as a base layer, and has its directionof magnetization fixed, and the other ferromagnetic layer is used as arecording layer. In this memory cell, data is recorded by making digitalinformation (“0”, “1”) correspond to the parallel or anti-parallelstate, concerning direction of magnetization, of the base layer andrecording layer. Writing of data to the memory cell is realized byswitching the direction of magnetization of the recording layer, using amagnetic field that is generated by a current guided to write wiringprovided for the memory cell.

Further, reading of data from the memory cell is realized by guiding acurrent to the ferromagnetic tunnel junction and detecting a change inthe resistance of the junction due to the TMR effect. A large number ofmemory cell similar to the above-described memory cell are arranged intoa magnetic memory. More specifically, to select an arbitrary cell, aswitching transistor is provided for each cell as in, for example, adynamic random access memory (DRAM), and a peripheral circuit isincorporated. Further, a device, in which a ferromagnetic tunneljunction and diode are provided at the intersection of a word line andbit line, has been proposed (see U.S. Pat. Nos. 5,640,343 and5,650,958).

To operate an MRAM using memory cells that include ferromagnetic tunneljunctions, it is always necessary to eliminate erroneous writing of datato non-selected cells. In MRAMs, a magnetic field acquired bysynthesizing magnetic field Hx, generated in the direction of the axisof easy magnetization, with magnetic field Hy generated in the directionof the axis of hard magnetization is applied to each selected cell,thereby writing data thereto. At this time, no magnetic field is appliedor only a one-directional magnetic field is applied to the non-selectedcells. Note that the memory cell to which only a one-directionalmagnetic field is applied is called an incompletely selected cell.

The magnetization switching characteristic in a simultaneous rotationmodel is expressed by the asteroid curve. As can be understood from theasteroid curve, switching magnetic field intensity Hsw necessary formagnetization switching when a magnetic field is applied in both thedirection of easy magnetization and the direction of hard magnetizationis lower than easy-axis switching magnetic field intensity Hc. At thistime, one-directional magnetic field intensity Hx necessary to writedata to a selected memory cell can be set to a value lower than Hc.Accordingly, theoretically, erroneous writing of data to anyincompletely selected memory cell can be prevented. Actually, however,variations in switching magnetic field intensity exist, which may causedata to be erroneously written to an incompletely selected memory cellunless Hsw is sufficiently lower than Hc.

On the other hand, since magnetic random access memories function asnon-volatile memories, they are required to hold record data in a stablemanner. There is a parameter called a thermal fluctuation constant andused as a target for reliably recording data for a long time. It isknown that the thermal fluctuation constant is proportional to thevolume of the recording layer and switching magnetic field intensityHsw. Therefore, if switching magnetic field intensity Hsw is reduced toreduce the rate of erroneous writing, the thermal stability is alsoreduced, with the result that data cannot be held for a long time.

In light of the above, to put a highly integrated magnetic memory intopractice, it is very important to propose a magnetoresistive effectelement capable of holding data for a long time by enhancing the thermalstability with switching magnetic field intensity Hsw reduced.

Further, Japanese Patent No. 3548036 discloses a technique, related tothe above, for correcting the magnetization pattern of amagnetoresistive element to enhance the write characteristics.

BRIEF SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided amagnetic memory device comprising:

a magnetoresistive element including a fixed layer having a direction ofmagnetization thereof fixed, a recording layer having a direction ofmagnetization thereof varied, and a non-magnetic layer interposedbetween the fixed layer and the recording layer; and

a first wiring layer extending in a first direction and configured togenerate a magnetic field for recording data in the magnetoresistiveelement,

wherein the recording layer includes:

a base portion extending in a second direction rotated from the firstdirection by an angle falling within a range of more than 0° to not morethan 20°, the base portion including a first side and a second sideopposing each other and extending in the second direction, the baseportion also including a third side and a fourth side opposing eachother; and

a first projection and a second projection outwardly projecting from thefirst side and the second side, respectively, in a third directionperpendicular to the second direction, the third side and the fourthside being inclined with respect to the third direction in the samerotational direction as a rotational direction in which the seconddirection is rotated.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a plan view illustrating a first example of an MTJ element 10;

FIG. 2 is a sectional view illustrating the MTJ element 10 shown in FIG.1;

FIG. 3 is a plan view illustrating an essential part of an MRAM with theMTJ element 10 shown in FIG. 1;

FIG. 4 is a graph illustrating the asteroid curve of the MTJ element 10shown in FIG. 3;

FIG. 5 is a plan view illustrating an essential part of an MRAM with theMTJ element 10 according to a first embodiment of the invention;

FIG. 6 is a graph illustrating the asteroid curve of the MTJ element 10shown in FIG. 5;

FIG. 7 is a graph illustrating the dependency of magnetic fieldintensity Hc of the MTJ element 10 upon the rotational angle;

FIG. 8 is a graph illustrating the dependency of variation σHc ofmagnetic field intensity Hc of the MTJ element 10 upon the rotationalangle;

FIG. 9 is a view illustrating specific values corresponding to the curveshown in FIG. 8;

FIG. 10 is a plan view illustrating a second example of the MTJ element10;

FIG. 11 is a plan view illustrating a third example of the MTJ element10;

FIG. 12 is a plan view illustrating a fourth example of MTJ element 10;

FIG. 13 is a plan view illustrating a fifth example of the MTJ element10;

FIG. 14 is a plan view illustrating a sixth example of MTJ element 10;

FIG. 15 is a circuit diagram illustrating a selective-transistor-typeMRAM according to a third embodiment of the invention;

FIG. 16 is a plan view illustrating the structure of theselective-transistor-type MRAM shown in FIG. 15;

FIG. 17 is a sectional view illustrating the structure of theselective-transistor-type MRAM shown in FIG. 15;

FIG. 18 is a circuit diagram illustrating a selective-diode-type MRAMaccording to the third embodiment of the invention;

FIG. 19 is a sectional view illustrating a memory cell MC incorporatedin the selective-diode-type MRAM of FIG. 18;

FIG. 20 is a circuit diagram illustrating a cross-point-type MRAMaccording to the third embodiment of the invention; and

FIG. 21 is a sectional view illustrating a memory cell MC incorporatedin the cross-point-type MRAM of FIG. 20.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention will be described with reference to theaccompanying drawings. In the description below, elements having thesame function and structure are denoted by the same reference numeral,and a duplicate explanation will be given only when necessary.

For reducing the degree of erroneous writing of data and enhancing theyield, the inventors of the present invention have produced, asdescribed blow, magnetic tunnel junction (MTJ) elements as examples ofmagnetoresistive effect elements.

FIG. 1 is a plan view illustrating a first example of an MTJ element 10.FIG. 2 is a sectional view illustrating the MTJ element 10 shown inFIG. 1. FIG. 2 is a sectional view taken, for example, in the Xdirection of FIG. 1.

As shown in FIGS. 1 and 2, the first example of the MTJ element 10 atleast comprises a fixed layer (also called a pinned layer) 11 having itsdirection of magnetization fixed, a recording layer (also called a freelayer) 13 having its direction of magnetization varied, and anon-magnetic layer (e.g., tunnel insulation layer) 12 interposed betweenthe fixed layer 11 and recording layer 13. An anti-ferromagnetic layer14, for example, is provided under the fixed layer 11 for fixing thedirection of magnetization of the fixed layer 11.

The MTJ element 10 is a so-called cross-shaped element formed of a baseportion 10 a extending in the X direction, and projections 10 b and 10 cprojecting from, for instance, central portions of opposed sides of thebase portion 10 a in the Y direction perpendicular to the X direction.In other words, in the planar configuration of the MTJ element 10, thewidth W1 of the element 10 at the center is greater than the width W2 atan end. Further, in FIG. 1, the length direction, i.e., the X direction,of the base portion 10 a is the direction of the axis of easymagnetization of the MTJ element 10, and the projection direction of theprojections 10 b and 10 c, i.e., the Y direction, is the direction ofthe axis of hard magnetization of the MTJ element 10. The direction ofthe easy magnetization is defined as the direction in which the magneticmoment of the ferromagnet is the easiest to point. The direction of thehard magnetization is defined as the direction in which the magneticmoment of the ferromagnet is the hardest to point. The axis of easymagnetization and that of hard magnetization will hereinafter bereferred to as “easy axis” and “hard axis”, respectively.

It is desirable that the projections 10 b and 10 c project from centralportions of opposed sides of the base portion 10 a in the Y direction.However, the invention is not limited to this. The projections 10 b and10 c may be located asymmetrically with respect to the central line ofthe base portion 10 a that extends in the X direction. Further, thecorners of the projections 10 b and 10 c may be rounded or angled.Namely, the asteroid curve of the MTJ element 10 is not significantlyinfluenced by the shape of each corner of the projection 10 b or 10 c.

The planar configuration of the base portion 10 is, for example, arectangle in which adjacent sides thereof do not intersect at rightangles. Namely, one pair of angles included in the two pairs ofdiagonally opposite angles, i.e., the angles of corners A and C, areacute, while the other pair of angles, i.e., the angles of corners B andD, are obtuse. The two opposite sides of the base portion 10 a, whichare provided with the projections 10 b and 10 c, are, for example,parallel with each other, and also parallel with the direction (Xdirection) of extension of the base portion 10 a.

It is not always necessary to make, parallel to each other, the otheropposite sides of the base portion 10 a, which are not provided with theprojection 10 b or 10 c. Namely, it is sufficient if these two sides areinclined in the same direction (i.e., at the same angle) with respect tothe hard axis. Further, the four corners A, B, C and D of the baseportion 10 a may be rounded or angled. In the MTJ element 10 of FIG. 1,the base portion 10 a has a planar configuration of a parallelogram.

The planar configuration of the MTJ element 10 has 180° rotationsymmetry (or two-rotation symmetry), and has no reflection symmetry.Although the fixed layer 11, non-magnetic layer 12 and recording layer13 of the MTJ element 10 shown in FIGS. 1 and 2 all have planarconfigurations, it is sufficient if at least the recording layer 13 hasthe above-described configuration.

The aspect ratio L/W of the MTJ element 10 is set to a value higher than1, and desirably to 1.5 to 2.2. The desirable value has been computed inlight of variations in switching magnetic field intensity Hc in thedirection of the easy axis. If the aspect ratio is set to the desirablevalue, the variations in switching magnetic field intensity Hc can besuppressed. For example, in the planar configuration as shown in FIG. 1,L of the aspect ratio L/W of the MTJ element 10 is defined as themaximum length in the X direction, and W of the aspect ratio L/W isdefined as the maximum length in the Y direction.

In the case of the MTJ element 10 of the shown configuration, the lengthL is the maximum distance between the X-directional opposite sides ofthe base portion 10 a measured in parallel with the X direction.Further, the width W is the maximum distance between the Y-directionalopposing sides of the projections 10 b and 10 c measured in parallelwith the Y direction. In other words, the length L is defined as thedistance acquired by connecting the midpoint Mab of the side between thecorners A and B to the midpoint Mcd of the side between the corners Cand D.

A description will now be given of an example of a material for the MTJelement 10. It is preferable to form the fixed layer 11 and recordinglayer 13 of, for example, Fe, Co or Ni, stacked layers of these metals,an alloy of these metals, magnetite of a high spin polarizability, anoxide such as CrO₂, RXMnO_(3-y) (R: rare metal, X: Ca, Ba, Sr), or aHeusler alloy such as NiMnSb or PtMnSb. These magnetic materials maycontain a small amount of a non-magnetic element, such as Ag, Cu, Au,Al, Mg, Si, Bi, Ta, B, C, O, N, Pd, Pt, Zr, Ir, W, Mo or Nb, as long asthe magnetic materials do not lose their magnetism.

It is preferable to form the anti-ferromagnetic layer 14 of, forexample, Fe—Mn, Pt—Mn, Pt—Cr—Mn, Ni—Mn, Ir—Mn, NiO or Fe₂O₃.

The non-magnetic layer 12 may be formed of a dielectric, such as Al₂O₃,SiO₂, MgO, AlN, Bi₂O₃, MgF₂, CaF₂, SrTiO₂ or AlLaO₃. These dielectricsmay have a loss of contain oxygen, nitrogen or fluoride.

The asteroid curve of the MTJ element 10 constructed as the above willbe described. FIG. 3 is a plan view illustrating an essential part of anMRAM with the MTJ element 10 of FIG. 1. As shown in FIG. 3, a firstwrite wiring layer (first write wiring) 15 extends in the X direction,and a second write wiring layer (second write wiring) 16 extends in theY direction perpendicular to the X direction. The first write wiring 15applies a Y-directional magnetic field to the MTJ element 10, and thesecond write wiring 16 applies an X-directional magnetic field to theMTJ element 10.

The MTJ element 10 is provided at the intersection of the first andsecond write wiring 15 and 16. More specifically, the easy axis of theMTJ element 10 is substantially parallel with the direction of extensionof the first write wiring 15. The hard axis of the MTJ element 10 issubstantially parallel with the direction of extension of the secondwrite wiring 16.

FIG. 4 is a graph illustrating the asteroid curve of the MTJ element 10shown in FIG. 3. Assume here that the write magnetic field (thesynthesis magnetic field of the easy-axis-directional magnetic field andhard-axis-directional magnetic field) necessary for magnetizationswitching at point P at which line L inclined by 45° with respect to theX axis intersects the asteroid curve is defined as switching magneticfield intensity Hsw. Assume also that the write magnetic field necessaryfor magnetization switching only in the easy-axis direction (i.e., themagnetic field intensity at a point at which the asteroid curveintersects the X axis) is defined as switching magnetic field intensityHc.

Magnetic field intensity Hsw necessary for magnetization switching whena magnetic field is applied in the easy-axis direction and hard-axisdirection is smaller than magnetic field intensity Hc necessary formagnetization switching when a magnetic field is applied only in theeasy-axis direction. At this time, since single-directional magneticfield intensity Hx necessary to write data to a selected memory cell canbe set smaller than magnetic field intensity Hc, erroneous writing ofdata to an incompletely selected memory cell does not theoreticallyoccur. However, in actual memory cells, variations exist in switchingmagnetic field intensity, therefore unless Hsw is set to a valuesufficiently lower than Hc, erroneous writing of data to theincompletely selected memory cell may well occur.

As is evident from FIG. 4, in such an MTJ element 10 as has theconfiguration shown in FIG. 1, Hc is sufficiently high with respect toHsw, and the write margin and thermal stability are enhanced.

However, if a close inspection is made on the portions of the asteroidcurve near the X axis, the asteroid curve is asymmetrical in the firstand second quadrants. Further, point Q, at which the asteroid curve isclosed when a Y-directional magnetic field is applied, is slightlydeviated from the Y axis. This will reduce the write margin and hence isnot preferable.

(First Embodiment)

FIG. 5 is a plan view illustrating an essential part of an MRAM with theabove-described MTJ element 10 according to a first embodiment of theinvention.

As shown, first write wiring 15 extends in the X direction, and secondwrite wiring 16 extends in the Y direction perpendicular to the Xdirection. The first write wiring 15 applies a Y-directional magneticfield to the MTJ element 10, and the second write wiring 16 applies anX-directional magnetic field to the MTJ element 10. The first and secondwrite wiring 15 and 16 may be provided separate by a preset distancefrom the MTJ element 10, or be electrically connected thereto.

The MTJ element 10 is provided at the intersection of the first andsecond write wiring 15 and 16. The easy axis of the MTJ element 10 isinclined by, for example, 5° with respect to the direction of extensionof the first write wiring 15 (i.e., the X direction). Similarly, thehard axis of the MTJ element 10 is inclined by, for example, 5° withrespect to the direction of extension of the second write wiring 16(i.e., the Y direction).

The direction of rotation of the MTJ element 10 is the same as thedirection in which the opposite sides of the base portion 10 a, whichare not provided with the projection 10 b or 10 c, are inclined withrespect to the hard axis (Y axis).

FIG. 6 shows the asteroid curve of the MTJ element 10 shown in FIG. 5.FIG. 6 also shows the asteroid curve of a comparative in which the easyaxis is substantially parallel with the first write wiring 15(rotational angle of 0°). Namely, FIG. 6 shows the asteroid curve of theMTJ element 10 shown in FIG. 3

As is evident from FIG. 6, when the asteroid curve of the MTJ element 10that is not inclined is compared with that of the MTJ element 10inclined by 5°, the degree of asymmetry near the X axis between thefirst and second quadrants is lower in the latter MTJ element 10 than inthe former MTJ element 10. Therefore, the write margin can be enhanced.

Further, point Q, at which the asteroid curve is closed when aY-directional magnetic field is applied, is substantially identical tothe Y axis in the latter MTJ element 10, whereas in the former MTJelement 10, the point is slightly deviated from the Y axis. Namely, thedegree of asymmetry near the Y axis between the first and secondquadrants is also lower in the latter MTJ element 10 than in the formerMTJ element 10.

A description will now be given of easy-axis switching magnetic fieldintensity Hc and variation σHc in Hc acquired when the rotational angleof the MTJ element 10 is changed.

FIG. 7 is a graph illustrating the dependency of magnetic fieldintensity Hc of the MTJ element 10 upon the rotational angle. Theabscissa indicates the rotational angle (°), and the ordinate indicatesthe ratio of Hc to Hc0. Hc0 indicates Hc acquired when the rotationalangle is 0°. Namely, in FIG. 7, magnetic field intensity Hc isstandardized by east-axis switching magnetic field intensity Hc0 of theMTJ element 10 acquired when the rotational angle is 0°.

As can be understood from FIG. 7, when the rotational angle of the MTJelement 10 is varied within the range of more than 0° to not more than20°, magnetic field intensity Hc is little changed. In other words, inthe range of more than 0° to not more than 20°, Hc is sufficiently highwith respect to Hsw, and hence the effect of enhancing the write marginand thermal stability is substantially the same as in the case where therotational angle is 0°.

FIG. 8 is a graph illustrating the dependency of variation σHc ofmagnetic field intensity Hc of the MTJ element 10 upon the rotationalangle. The abscissa indicates the rotational angle (°), and the ordinateindicates the ratio of σHc to σHc0. σHc0 indicates σHc acquired when therotational angle is 0°. Namely, in FIG. 8, variation σHc is standardizedby variation σHc0 of the MTJ element 10 acquired when the rotationalangle is 0°. FIG. 9 shows specific values of σHc/σHc0 corresponding tothe rotational angles 0°, 2°, 5°, 10° and 20°.

As can be understood from FIG. 8, as the rotational angle of the MTJelement 10 is increased, σHc/σHc0 is reduced. Further, when therotational angle falls within a range of more than 0° to not more than20, σHc/σHc0 is abruptly reduced, and it is minimum when the rotationalangle is 2°.

When σHc/σHc0 is lower than 1, the write margin for reducing variationσHc is increased. Namely, if the rotational angle falls within the rangeof more than 0° to not more than 20°, σHc can be reduced.

Further, if the rotational angle falls within the range of more than 0°to not more than 15° , σHc can be reduced by 10% or more. Accordingly,in this range, the effect of reducing variation σHc is significant.

In particular, when the rotational angle is 10°, σHc assumes a value(specifically, 0.88) lower by more than 10% than the value acquired whenthe rotational angle is 0°. When the rotational angle is 5°, σHc isreduced up to 0.87. In light of this, it is most desirable to set therotational angle to more than 0° to not more than 5°.

As described above in detail, in the embodiment, X-directional switchingmagnetic field intensity Hc can be set to a sufficiently high value withrespect to Hsw, with the result that the write margin and thermalstability can be enhanced, thereby reducing the degree of erroneouswriting of data to the MTJ element 10.

Furthermore, variation σHc in magnetic field intensity Hc can be reducedwithout reducing magnetic field intensity Hc, which enhances the yieldof MTJ elements 10.

Note that when the inclination (or rotational angle), with respect tothe hard axis, of the opposite sides of the base portion 10 a, which areprovided with no projections, is changed, Hsw of the asteroid curve isalso changed. However, even if the rotational angle of those sides ischanged, the asymmetry of the asteroid curve is little changed.Accordingly, even if the embodiment is applied to the MTJ element 10 inwhich the rotational angle of those opposite sides is changed, the sameadvantage as the above can be acquired.

Also, as described above, the MTJ element 10 may be constructed suchthat only the recording layer 13 has the base portion 10 a andprojections 10 b and 10 c. Namely, the asteroid curve of the MTJ element10 is substantially determined from the planar configuration of therecording layer 13. Therefore, the planar configuration of the fixedlayer 11, non-magnetic layer 12 or anti-ferromagnetic layer 14 is notlimited to the above-mentioned one, but may be, for example, arectangle.

(Second Embodiment)

The MTJ element 10 used in the present invention is not limited to theconfiguration employed in the first embodiment, but may be modified invarious ways. An MTJ element 10 according to a second embodiment hasanother configuration. Also in the second embodiment, it is a matter ofcourse that the easy axis can be rotated within the range of more than0° to not more than 20° with respect to the first write wiring 15.

FIG. 10 is a plan view illustrating a second example of the MTJ element10 according to the second embodiment. As shown, the base portion 10 ahas a shape of, for example, a parallelogram, and the projections 10 band 10 c are rounded. The projections 10 b and 10 c may have roundedcorners, or may be entirely rounded as shown in FIG. 10. Namely, thedegree of rounding (more specifically, curvature radius R) is notlimited to a particular value. Alternatively, the projections 10b and 10c may have linearly cut-away corners. The aspect ratio of the MTJelement of FIG. 10 is identical to that of the MTJ element of FIG. 1.

FIG. 11 is a plan view illustrating a third example of the MTJ element10. The MTJ element of FIG. 11 is acquired by reducing the aspect ratioof the MTJ element of FIG. 10. However, the former also satisfies thecondition that the aspect ratio is not less than 1. Thus, an MTJ element10 having a low aspect ratio can also be used.

FIG. 12 is a plan view illustrating a fourth example of the MTJ element10. The base portion 10 a of the fourth example has rounded corners. Thecurvature radius of each corner of the fourth example is not limited toany particular value. Alternatively, the base portion 10 a may havelinearly cut-away corners, instead rounded corners. The projections 10 band 10 c of the fourth example are rounded. Alternatively, they may haveangled corners. Actually, from the etching accuracy in the manufacturingprocess, it is difficult to form such an MTJ element 10 as shown in FIG.1, in which all corners are angled. It is easier to produce such an MTJelement as shown in FIG. 12, and hence it is strongly possible that suchan MTJ element as shown in FIG. 12 is actually employed. Further, thelength L of the MTJ element shown in FIG. 12 is defined as the distancebetween the midpoint Mab of the side AB and the midpoint Mcd of the sideCD.

FIG. 13 is a plan view illustrating a fifth example of the MTJ element10. The opposite sides of the base portion 10 a, which are not providedwith the projection 10 b or 10 c, are inclined in the same direction atdifferent angles with respect to the hard axis.

In other words, one pair of angles included in the two pairs ofdiagonally opposite angles, i.e., the angles of corners A and C, aredifferent acute angles, while the other pair of angles, i.e., the anglesof corners B and D, are different obtuse angles. The corners of the baseportion 10 a are, for example, rounded. The projections 10 b and 10 care, for example, rounded.

FIG. 14 is a plan view illustrating a sixth example of the MTJ element10. Two corners A and C of the base portion 10 a corresponding to acutecorners have different curvature radiuses. Namely, corner A, included incorners A and C corresponding to the acute angles, has a smaller radiusof curvature than corner C. The projections 10 b and 10 c are, forexample, rounded.

Also when the above-mentioned MTJ elements 10 are employed, and evenwhen they are arranged with the angle thereof rotated with respect tothe write wiring 15 and 16, the same advantage as in the firstembodiment can be acquired. It is a matter of course that only therecording layer 13 in each MTJ element 10 may have the above-describedconfiguration.

A method for manufacturing such an MTJ element 10 as the above will nowbe described.

[1] Manufacturing Method Example 1

In a manufacturing method example 1, an MTJ element 10 of a standardsize is manufactured.

Firstly, an MTJ material layer is formed by sputtering, and a resist iscoated on the MTJ material layer. Subsequently, a pattern is formed inthe layer, using light, an electron beam or X-rays, and is developedinto a resist pattern. Using the resist pattern as a mask, the MTJmaterial layer is subjected to ion milling or etching, thereby formingan MTJ element 10 of a desired configuration. After that, the resistlayer is removed.

[2] Manufacturing Method Example 2

In a manufacturing method example 2, an MTJ element 10 of a relativelylarge size, e.g., of a micron-order size, is manufactured.

Firstly, an MTJ material layer is formed by sputtering, and a hard maskformed of, for example, silicon oxide or silicon nitride, is formed. Thehard mask is etched into a hard mask pattern of a desired configuration,using reactive ion etching (RIE). Using this hard mask pattern, the MTJmaterial layer is subjected to ion milling to form an MTJ element 10 ofa desired configuration.

[3] Manufacturing Method Example 3

In a manufacturing method example 3, an MTJ element 10 of a relativelysmall size, e.g., of a sub-micron-order size ranging from about 2-3 μmto about 0.1 μm, is manufactured. For manufacturing an MTJ element ofsuch a size as this, photolithography can be utilized as follows:

Firstly, an MTJ material layer is formed by sputtering, and a hard maskformed of, for example, silicon oxide or silicon nitride, is formed. Thehard mask is etched into a hard mask pattern of a desired configuration,using photolithography. Using this hard mask pattern as a mask, the MTJmaterial layer is subjected to RIE to form an MTJ element 10 of adesired configuration.

[4] Manufacturing Method Example 4

In a manufacturing method example 4, an MTJ element 10 of a yet smallersize, e.g., of about 0.5 μm or less, is manufactured. For manufacturingan MTJ element of such a size as this, electron beam exposure can beutilized.

However, in this case, since the element is very small, the part of theelement that is used for increasing the edged domain region employed inan embodiment of the invention is extremely small. Accordingly, it isvery difficult to manufacture such an MTJ element.

In light of this, to manufacture an MTJ element 10 of a desiredconfiguration according to the embodiment, optical proximity correctionof an electron beam is utilized. The optical proximity correction methodis used to correct a proximity effect that occurs in the pattern due torearward scatter of an electron beam from the substrate, thereby forminga correct pattern. Optical proximity correction is executed, forexample, as follows. When, for example, a rectangular pattern is formed,a phenomenon occurs in which only a small amount of charge isaccumulated near each corner of the rectangular pattern, and hence eachcorner is rounded. To sharpen each corner, a correction point beam isapplied to the portion of the pattern near each corner, e.g., theportion outside the pattern in the case of, in particular, an element ofabout 0.5 μm or less, thereby increasing the amount of the accumulatedcharge to acquire a correct pattern.

In the manufacturing method example 4, using the above-describedproximity effect correction of an electron beam, a configuration inwhich element corners are formed wide is acquired in the followingmanner. For instance, when a so-called cross-shaped element is formed, acorrection point beam is applied to each of the opposite portions of theelement where projections are to be formed. As a result, wideprojections are formed at the opposite sides of the element. At thistime, it is advisable to correct the configuration of the element so asto, for example, sharpen each corner by applying thereto a larger amountof charge, and/or more appropriately adjusting the correction point beamapplication position, than in the case of standard proximity effectcorrection. Furthermore, to form a so-called cross-shaped pattern, aplurality of correction point beams may be applied.

(Third Embodiment)

A third embodiment is directed to an MRAM structure example using theabove-described MTJ element 10.

It is preferable to use the above-described MTJ element 10 as a memoryelement for a memory cell incorporated in an MRAM. In general, an MRAMusing a magnetic member as a recording layer requires a thermally stablerecording layer that is free from erroneous writing of data to adjacentcells, and can hold record data for a long time even if memory cells arereduced in size. If the MTJ element according to an embodiment is used,a memory cell having a low switching magnetic field intensity andsufficiently high thermal fluctuation constant can be provided. As aresult, the current needed for writing a record bit can be reduced.

A description will now be given of each of the memory cell structuresfor an MRAM, i.e., [1] selective transistor type, [2] selective diodetype, and [3] cross-point type.

[1] Selective Transistor Type

FIG. 15 is a circuit diagram illustrating a selective-transistor-typeMRAM according to the third embodiment of the invention. FIG. 16 is aplan view illustrating the structure of the selective-transistor-typeMRAM shown in FIG. 15. FIG. 17 is a sectional view illustrating thestructure of a memory cell MC incorporated in theselective-transistor-type MRAM shown in FIG. 15. For facilitating thedescription, FIG. 16 shows only a bit line (BL), word line (WWL) and MTJelement 10. In the structure of FIG. 17, an interlayer insulation layer(not shown) is filled between a semiconductor substrate 21 and bit line(BL) 28.

As shown, the selective-transistor-type memory cell MC comprises a MTJelement 10, transistor Tr (e.g., metal oxide semiconductor (MOS)transistor) electrically connected to the MTJ element 10, bit line (BL)28 and word line (WWL) 26. Memory cells MC similar to this cell arearranged in an array, thereby providing a memory cell array MCA. Theword line (WWL) 26 corresponds to the first write wiring 15 shown inFIG. 5, and the bit line (BL) 28 corresponds to the second write wiring16 shown in FIG. 5.

Specifically, an end of the MTJ element 10 is electrically connected toan end (drain diffusion region) 23 a of the current path of thetransistor Tr via a base metal layer 27, contact plugs 24 a, 24 b and 24c and wiring layers 25 a and 25 b. The other end of the MTJ element 10is electrically connected to the bit line 28. The write word line (WWL)26 is provided below the MTJ element 10. The write word line (WWL) 26may be electrically connected to or disconnected from the MTJ element10. The other end (source diffusion region) 23 b of the current path ofthe transistor Tr is, for example, grounded via a contact plug 24 d andwiring layer 25 c. The gate electrode 22 of the transistor Tr functionsas a read word line (RWL).

The end of the MTJ element 10 coupled to the base metal layer 27 isformed of, for example, the fixed layer 11, and the other end of the MTJelement 10 coupled to the bit line 28 is formed of, for example, therecording layer 13, but may be vice versa. Further, a hard mask may beinterposed between the MTJ element 10 and bit line 28.

As shown in FIG. 16, the MTJ element 10 has its easy axis inclined by,for example, 5° with respect to the direction of extension of the wordline (WWL) 26, i.e., the X direction. In other words, the MTJ element 10has its hard axis inclined by, for example, 5° with respect to thedirection of extension of the bit line (BL) 28, i.e., the Y direction.Further, the MTJ element 10 has, for example, such a planarconfiguration as shown in FIG. 12.

Data is written to and read from the selective-transistor-type memorycell MC, constructed as the above, as follows:

<Writing Operation>

The bit line 28 and write word line 26 corresponding to a selected oneof the MTJ elements are selected (activated). When write currents Iw1and Iw2 are guided to the selected bit line 28 and write word line 26,respectively, a synthesis magnetic field due to the write currents Iw1and Iw2 is applied to the selected MTJ element 10. As a result, thedirection of magnetization of the recording layer 13 incorporated in theMTJ element 10 is switched, and the direction of magnetization of thefixed layer 11 becomes parallel or anti-parallel with that of therecording layer 13. If the parallel state and anti-parallel stateconcerning the direction of magnetization are defined as “1” and “0”,respectively, writing of digital data can be realized.

<Reading Operation>

Data reading is executed as follows, using the transistor Tr thatfunctions as a switching element for reading. The bit line 28 and readword line (RWL) 22 corresponding to a selected one of the MTJ elementsare selected (activated), thereby guiding, to the MTJ element 10, a readcurrent Ir that passes through the non-magnetic layer 12 by tunneling.At this time, the junction resistance is varied in accordance with thecosine corresponding to the angle between the direction of magnetizationof the fixed layer 11 and that of the recording layer 13, therebyproviding a tunnel magnetic resistance (TMR) effect. The TMR effect is aphenomenon in which if the direction of magnetization of the fixed layer11 is parallel with that of the recording layer 13 (i.e., the data is“1”), the junction resistance is low, whereas if the former isanti-parallel with the latter (i.e., the data is “0”), the junctionresistance is high. Thus, the state “1” or “0” of the MTJ element 10 canbe determined by detecting the junction resistance.

[2] Selective Diode Type

FIG. 18 is a circuit diagram illustrating a selective-diode-type MRAMaccording to the third embodiment of the invention. FIG. 19 is asectional view illustrating a memory cell MC incorporated in theselective-diode-type MRAM of FIG. 18. In the example of FIG. 18, ashallow trench isolation (STI) layer and interlayer insulation layer(not shown) are filled between the semiconductor substrate 21 and bitline (BL) 28.

As shown in FIGS. 18 and 19, the selective-diode-type memory cell MCcomprises a MTJ element 10, diode D electrically connected to the MTJelement 10, bit line (BL) 28 and word line (WWL) 26. Memory cells MCsimilar to this cell are arranged in an array, thereby providing amemory cell array MCA.

The diode D is, for example, a pn-junction diode that comprises a p-typesemiconductor layer and n-type semiconductor layer. An end (e.g., thep-type semiconductor layer) of the diode D is electrically connected tothe MTJ element 10. The other end (e.g., the n-type semiconductor layer)of the diode D is electrically connected to the word line 26. In theshown structure, a current flows from the bit line 28 to the word line26.

The location and orientation of the diode D can be varied. For instance,the orientation of the diode D may be set to guide a current from theword line 26 to the bit line 28. Further, the diode D may be formed inthe semiconductor substrate 21. The diode D may have the sameconfiguration (e.g., a so-called cross shape) as the MTJ element 10.

The MTJ element 10 has its easy axis inclined by, for example, 5° withrespect to the direction of extension of the word line (WL) 26, i.e.,the X direction. In other words, the MTJ element 10 has its hard axisinclined by, for example, 5° with respect to the direction of extensionof the bit line (BL) 28, i.e., the Y direction. Further, the MTJ element10 has, for example, such a planar configuration as shown in FIG. 12.

In the selective-diode-type memory cell MC constructed as the above, theoperation of writing data is similar to that in theselective-transistor-type memory cell MC. Namely, currents Iw1 and Iw2are supplied to the bit line 28 and word line 26, respectively, therebymaking the direction of magnetization of the recording layer 13 and thatof fixed layer 11 of the MTJ element 10 parallel with or anti-parallelwith each other.

The operation of reading data is also substantially the same as in theselective-transistor-type memory cell MC. However, in theselective-diode-type memory cell MC, the diode D is used as a switchingelement. Namely, the rectifying function of the diode D is utilized tocontrol the bit lines 28 and word lines 26 to apply a reverse bias tonon-selected MTJ elements, thereby causing current Ir to flow into onlya selected MTJ element 10.

[3] Cross-point Type

FIG. 20 is a circuit diagram illustrating a cross-point-type MRAMaccording to the third embodiment of the invention. FIG. 21 is asectional view illustrating a memory cell MC incorporated in thecross-point-type MRAM of FIG. 20. In the example of FIG. 20, a shallowtrench isolation (STI) layer and interlayer insulation layer (not shown)are filled between the semiconductor substrate 21 and bit line (BL) 28.

As shown in FIGS. 20 and 21, the cross-point type memory cell MCcomprises a MTJ element 10, bit line (BL) 28 and word line (WL) 26.Memory cells MC similar to this cell are arranged in an array, therebyproviding a memory cell array MCA.

Specifically, the MTJ element 10 is positioned near the intersection ofthe bit line 28 and word line 26. An end of the MTJ element 10 iselectrically connected to the word line 26, and the other end of theelement 10 is electrically connected to the bit line 28.

The MTJ element 10 has its easy axis inclined by, for example, 5° withrespect to the direction of extension of the word line (WL) 26, i.e.,the X direction. In other words, the MTJ element 10 has its hard axisinclined by, for example, 5° with respect to the direction of extensionof the bit line (BL) 28, i.e., the Y direction. Further, the MTJ element10 has, for example, such a planar configuration as shown in FIG. 12.

In the cross-point type memory cell MC constructed as the above, theoperation of writing data is similar to that in theselective-transistor-type memory cell MC. Namely, currents Iw1 and Iw2are supplied to the bit line 28 and word line 26, respectively, therebymaking the direction of magnetization of the recording layer 13 and thatof fixed layer 11 of the MTJ element 10 parallel with or anti-parallelwith each other. In contrast, to read data from a selected MTJ element10, read current Ir is supplied to the bit line 28 and word line 26connected to the selected MTJ element 10.

As described above in detail, a plurality of types of magnetic randomaccess memories (MRAM) can be constructed using MTJ elements 10. Theabove-described MRAMs types are just examples, and the present inventionis also applicable to other types of MRAMs.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A magnetic memory device comprising: a magnetoresistive elementincluding a fixed layer having a direction of magnetization thereoffixed, a recording layer having a direction of magnetization thereofvaried, and a non-magnetic layer interposed between the fixed layer andthe recording layer; and a first wiring layer extending in a firstdirection and configured to generate a magnetic field for recording datain the magnetoresistive element, wherein the recording layer includes: abase portion extending in a second direction rotated from the firstdirection by an angle falling within a range of more than 0° to not morethan 20°, the base portion including a first side and a second sideopposing each other and extending in the second direction, the baseportion also including a third side and a fourth side opposing eachother; and a first projection and a second projection outwardlyprojecting from the first side and the second side, respectively, in athird direction perpendicular to the second direction, the thirdside-and the fourth side being inclined with respect to the thirddirection in the same rotational direction as a rotational direction inwhich the second direction is rotated.
 2. The magnetic memory deviceaccording to claim 1, wherein: the second direction corresponds to anaxis of easy magnetization; and the third direction corresponds to anaxis of hard magnetization.
 3. The magnetic memory device according toclaim 1, wherein the third direction is inclined, by an angle fallingwithin a range of more than 0° to not more than 20°, with respect to afourth direction perpendicular to the first direction.
 4. The magneticmemory device according to claim 1, wherein the recording layer has aplanar configuration which has 180° rotation symmetry and no reflectionsymmetry.
 5. The magnetic memory device according to claim 1, whereinthe base portion has a rectangular planar configuration which includes afirst pair of diagonally opposite two corners, and a second pair ofdiagonally opposite two corners, the first pair of corners having anacute angle, the second pair of corners having an obtuse angle.
 6. Themagnetic memory device according to claim 1, wherein the base portionhas a planar configuration of a parallelogram.
 7. The magnetic memorydevice according to claim 1, wherein a length of the recording layer inthe second direction is longer than a length of the recording layer inthe third direction.
 8. The magnetic memory device according to claim 1,wherein a length of the recording layer in the second direction is 1.5times or more a length of the recording layer in the third direction. 9.The magnetic memory device according to claim 1, wherein the recordinglayer has a plurality of round corners.
 10. The magnetic memory deviceaccording to claim 1, wherein the first projection has two roundcorners, and the second projection has two round corners.
 11. Themagnetic memory device according to claim 1, wherein the base portionhas four round corners.
 12. The magnetic memory device according toclaim 1, wherein the angle falls within a range of more than 0° to notmore than 15°.
 13. The magnetic memory device according to claim 1,wherein the angle falls within a range of more than 0° to not more than5°.
 14. The magnetic memory device according to claim 1, furthercomprising a second wiring layer extending in a fourth directionperpendicular to the first direction and configured to generate amagnetic field for recording the data in the magnetoresistive element.15. The magnetic memory device according to claim 1, wherein the fixedlayer includes a ferromagnetic layer and an anti-ferromagnetic layer.16. The magnetic memory device according to claim 1, further comprising:a bit line extending in a fourth direction perpendicular to the firstdirection, and electrically connected to a terminal of themagnetoresistive element; and a switching element having a terminalelectrically connected to another terminal of the magnetoresistiveelement.
 17. The magnetic memory device according to claim 16, furthercomprising a word line configured to control an on-state and anoff-state of the switching element.
 18. The magnetic memory deviceaccording to claim 16, further comprising a power supply connected toanother terminal of the switching element.
 19. The magnetic memorydevice according to claim 16, wherein the switching element is an MIS(Metal Insulator Semiconductor) transistor.